British Patent Specification No. 1,537,930 discloses such an apparatus which, like the present invention, primarily is intended for anti-corrosion treatment of sheet metal made of a zirconium alloy, for example Zircaloy-4. During such heat treatment, the sheet first is heated to a temperature of at least 800.degree. C. and then rapidly cooled in such a way that, for example, a heated sheet zone is subjected to a temperature reduction of at least 200.degree. C. within a maximum time of 60 seconds. When using the apparatus disclosed in the previously mentioned British patent, it has been found that it is difficult to avoid, to a sufficiently great extent, oxidation of the sheet material and deformation caused by temperature stresses.
German laid-open Application No. 1,172,050 also discloses such an apparatus; however it is limited since the sheet material must be coiled when inserted and withdrawn from the apparatus. Furthermore, the uncoiling, the heat treating, and recoiling after heat treating must be carried out in an evacuated container. In order to use an apparatus similar to the one shown in the previously mentioned German application for plane sheet material, it would be necessary to increase the length of the evacuated container by at least 100%. Further, it would be necessary to open the container and pump a new vacuum for each sheet length treated.